(WIP) Tests for PLUS impact algorithm

Work in progress; PR for discussion only.

<b>Status</b>
Position projection algorithm can be run in <i>exhaustive search</i> and <i>successive pruning</i> modes; impact handler currently performs exhaustive search only. Handlers no longer create internal copies of the MultibodySystem. Results seem reasonable for the six tests run here, though more testing is necessary. Toggle the <i>PRINT_</i> flags to see detailed output.

<b>To do</b>
- [x] Implement <i>successive pruning</i> mode for impact handler.
- [x] Add BodyWatcher so the camera tracks the brick.
- [x] Improve algorithm for determining the interval step length.
- [x] Use unique indices to access Array_ elements.
- [x] Add pause _before_ closing each visualization window (requested by @scpeters).
- [x] Print total system energy to monitor energy conservation (requested by @hsu).
- [x] Remove [`minIntervalsPerPhase`](https://github.com/tkuchida/simbody/blob/PLUSImpact/Simbody/tests/adhoc/TestPLUS_SingleBrick.cpp#L86) parameter (limiting [`maxSlidingDirChange`](https://github.com/tkuchida/simbody/blob/PLUSImpact/Simbody/tests/adhoc/TestPLUS_SingleBrick.cpp#L81) is sufficient).
- [ ] Add a mode that compares the exhaustive search and successive pruning results for each interval.
- [ ] Solve only for impulses in least-squares problem to improve efficiency.
- [ ] During restitution, remove equations with no degrees of freedom.
- [ ] Modify `GM\~G` computation used by Simbody; create `G'M\~G`, where `G'` is modified version of `G`.
- [ ] If all sliding directions are acceptable, do not update them (i.e., do not solve the linear system a second time).
- [ ] Investigate use of Newton iteration to refine sliding directions.
- [x] Improve handling of unknown slip directions in `calculateIntervalStepLength()`.
- [x] Improve efficiency of search strategy used in `calculateIntervalStepLength()`.
- [ ] Test in more scenarios and with different parameters.
- [ ] Compare results with compliant contact model.
- [ ] Generalize for arbitrary multibody systems by overloading MultibodySystem.
- [ ] Add contact handler.